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B12.5 More about Genetics

GCSE Combined Science · AQA · Chapter B12: Reproduction

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: sex determination, and using family trees to work out how characteristics are inherited. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Recall that human body cells have 23 pairs of chromosomes, one pair of which are sex chromosomes
  • Carry out a genetic cross to show how sex is inherited
  • Read a family tree and decide whether a characteristic is caused by a dominant or a recessive allele
  • Work out genotypes and probabilities from family trees using simple ratios

Key words

  • Sex chromosomes: The pair of chromosomes that determines sex: XX in females, XY in males.
  • X chromosome: The larger sex chromosome. Females have two; males have one.
  • Y chromosome: The smaller sex chromosome, found only in males.
  • Family tree: A diagram that shows how a characteristic has been passed through generations of a family.
  • Carrier: A person who is heterozygous: they carry one recessive allele but do not show the characteristic.

Chromosomes and sex

Ordinary human body cells contain 23 pairs of chromosomes. 22 pairs control characteristics only. One pair, the sex chromosomes, carries the genes that determine sex.

  • Female; Sex chromosomes in body cells XX; Sex chromosome in gametes every egg has an X
  • Male; Sex chromosomes in body cells XY; Sex chromosome in gametes half the sperm carry X, half carry Y

Key fact: It is the sperm that decides the sex of the baby: an X sperm gives a girl (XX); a Y sperm gives a boy (XY).

A genetic cross for sex

Treat the sex chromosomes just like alleles: the father's gametes across the top, the mother's down the side.

Worked example: probability of a girl

From the square: 2 out of 4 offspring are XX. Probability of a girl = 2/4 = 1/2 = 50%. Ratio 1 girl : 1 boy.
A couple already have two sons. The probability that the next baby is a girl is still 50%: every fertilisation is a separate event.

Answer: 0.5 (50%) every time

Worked example: three in a row (maths skill)

What is the probability that a couple's next three children are all boys?
Multiply the separate probabilities: 1/2 × 1/2 × 1/2 = 1/8.

Answer: 1/8 (12.5%)

Reading family trees

A family tree uses squares for males and circles for females. Shaded symbols show people with the characteristic. A horizontal line joins two parents; a line down leads to their children. Each generation is on its own row, oldest at the top.

Worked example: dominant or recessive?

Look for two parents who are the same with a child who is different. People 1 and 2 can roll their tongues, but their son 6 cannot.
So 1 and 2 must each carry a hidden allele. A hidden allele is recessive.
Using T (can roll) and t (cannot roll): 6 is tt, so 1 and 2 are both Tt.

Answer: not being able to roll is caused by a recessive allele

Exam tip: Two unaffected parents with an affected child → the condition is recessive. Two affected parents with an unaffected child → it is dominant.

Probabilities from a family tree

Worked example: the next child

People 7 and 8 can roll their tongues, but their daughter 9 cannot (tt). So 7 and 8 are both Tt.
Tt × Tt gives TT, Tt, Tt, tt. Probability that their next child cannot roll = 1/4 = 25%.
Probability that the next child is a boy who cannot roll = 1/4 × 1/2 = 1/8.

Answer: 25%; 1/8 for a non-rolling boy

Person 8 can roll his tongue, but his father (3) cannot. Person 3 is tt, so he could only pass on t: person 8 must be Tt. This agrees with the answer above.

Exam tip: In a recessive condition, an unaffected person who has an affected parent or an affected child must be heterozygous.

Common mistakes

  • Saying the mother decides the sex of the baby. All eggs carry X; the sperm carries X or Y.
  • Thinking that after several boys a girl is "due". Each baby has a 50% chance of being a girl.
  • Deciding a characteristic is dominant because many people have it. Use the pattern of parents and children.
  • Forgetting that an unaffected parent of an affected child (recessive characteristic) must be heterozygous.

Revision questions

Try these, then check your answers below each one.

Question 1. What are the sex chromosomes in a human female body cell? (Choose from: XX, XY, YY, X only.)

Answer: XX

Working: Females are XX and males are XY.

Question 2. True or false? The sex of a baby is decided by whether the sperm carries an X or a Y chromosome.

Answer: True

Working: All eggs carry X. Sperm carry X or Y, so the sperm decides: X gives a girl (XX), Y gives a boy (XY).

Question 3. A hospital records 1 860 births in a year. 948 of the babies are boys. What percentage of the babies are boys? Give your answer to 1 decimal place.

Answer: 51.0 %

Working: 948 ÷ 1860 × 100 = 50.97… = 51.0%. This is close to the 50% predicted by the genetic cross.

Related lessons

Continue in the full course

This lesson is section B12.5 (AGCSC1205) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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